stp: full RSTP configuration (bridge + per-port), CLI + GUI + persistence

Implements the standard 802.1D-2004/802.1w configuration surface:

Bridge:  priority (0-15 x4096), hello time, max age, forward delay,
         force-version (RSTP v2 / STP-compatible v0 Config BPDUs), tx hold
         count (per-port per-second BPDU budget).
Per port: enable, admin edge (forwarding immediately - no listen gap),
         auto edge (forwarding after 3 s of BPDU silence; DEFAULT, so
         host-facing ports no longer take the full forward delay),
         path cost (0=auto/20000), port priority, BPDU guard (port disabled
         on BPDU receipt), root guard (never accept a better root on the
         port), BPDU filter (no BPDUs in or out).

Engine additions: root max-age expiry (reclaim the tree when the root goes
silent), root path cost accounting (rx cost + root-port cost, advertised in
our BPDUs), loop detection (our own BPDU coming back blocks the port for a
listen period), topology-change counter, approximated per-port roles
(Root/Designated/Alternate) for diagnostics.

CLI: "stp prio|hello|maxage|fwd|txhold|version ..." and
"stp port <n> on|off|edge|cost|prio|guard|filter ..." (stp_parse, delegated
from cmd_parser); all forms accepted by the startup-config validator so the
whole configuration persists. /stp.json now reports config + status; the
Spanning Tree page exposes everything with immediate-apply controls and live
state/role columns (edit-in-flight guard against the 2 s refresh).

8051 memory: the module moves to code BANK2; internal-RAM pressure from
cross-bank calls resolved by xdata loop iterators/scratch, __reentrant on
the small helpers, and moving httpd's header-pointer globals to xdata.

Verified on hardware (SWTGW218AS): defaults land per standard; priority and
hello change live; admin-edge ports (the LACP bond uplinks) keep the LAN at
0% loss THROUGH "stp on"; auto-edge ports forward after 3 s; a port that
heard real BPDUs (a VM bridge behind physical port 6) correctly declined
auto-edge, sat out the full listen period and became Designated; tc counts
promotions; we win the root election at priority 16384 vs 32768.

(cherry picked from commit 09a34dc6acdc81ab9cab0727d2f4a59c68131a3e)

html/stp.html

@@ -13,7 +13,27 @@
     <h2>Mode: <select id="stpMode"><option value="off">Disabled</option><option value="on">Enabled</option></select>
       <input style="width:15%;margin-left: 3em;" class="action" id="stp_sub" onclick="stpSub();" type="button" value="Apply"></h2>
     <div id="stpStat" style="font-family:monospace;"></div>
-    <div id="stpPorts" style="font-family:monospace;white-space:pre;"></div>
+    <h2>Bridge settings</h2>
+    <table id="stpBridge">
+      <tr>
+        <th>Priority</th><th>Version</th><th>Hello [s]</th><th>Max age [s]</th><th>Fwd delay [s]</th><th>Tx hold</th>
+      </tr>
+      <tr>
+        <td><select id="bPrio"></select></td>
+        <td><select id="bVer"><option value="rstp">RSTP</option><option value="stp">STP compat</option></select></td>
+        <td><input id="bHello" type="number" min="1" max="10" style="width:4em"></td>
+        <td><input id="bMaxage" type="number" min="6" max="40" style="width:4em"></td>
+        <td><input id="bFwd" type="number" min="4" max="30" style="width:4em"></td>
+        <td><input id="bTxhold" type="number" min="1" max="10" style="width:4em"></td>
+      </tr>
+    </table>
+    <p style="font-size:small">Changes apply immediately. Edge ports skip the listen period; guard/filter act on received BPDUs.</p>
+    <h2>Ports</h2>
+    <table id="stpPortsTbl">
+      <tr>
+        <th>Port</th><th>State</th><th>Role</th><th>STP</th><th>Edge</th><th>Cost [k]</th><th>Priority</th><th>Guard</th><th>Filter</th>
+      </tr>
+    </table>
     <script src="/stp.js"></script>
     </div>
   </body>